The subject matter of the present invention is a method of preparing 2-phenylethylchlorosilanes in which hydrogen chlorosilanes are added onto styrene in the presence of platinum catalysts. The process suppresses the formation of the 1-phenylethyl compounds.
The addition of unsaturated organic compounds onto silicon compounds having one or two Si-H bonds in the presence of platinum catalysts is generally known (cf. J. Am. Ch. Soc. 79, 1957 pp. 974-979). In this reaction, also known as an addition reaction, when unsaturated organic compounds having multiple terminal bonds are used, the silyl moiety is mostly added onto the terminal carbon atom; however, to a more or less great extent an addition of the silyl moiety onto the adjacent carbon atom takes place, reducing the yields of the compound that is mainly desired, which contains the silyl moiety in the terminal position.
In the reaction of styrene with Si-H compounds, there is also the circumstance that, under the reaction conditions, a partial polymerization of the styrene to polystyrene easily takes place, which again reduces the yield of phenylethyl silanes. This tendency to polymerize is especially great when platinum compounds are used on active carbon as the support material.
It is for this reason that a number of attempts to avoid these problems have been described. However, any heterogeneous systems in which the formation of undesired 1-phenylethyl silane is suppressed, still have the disadvantage of an excessive formation of styrene polymers.
On the other hand, the procedures in which the formation of styrene polymers is suppressed, such as for example the use of platinum or rhodium complex compounds as catalysts and/or the use of certain solvents, have not as yet offered a satisfactory solution of the problems described.
In German Pat. No. 2,602,171, the addition of phenothiazine and other nitrogenous compounds to the reaction mixture to serve as promoters of the addition reaction has been described; it is stated therein that, when this compound is used in the reaction of styrene with trichlorosilane, the beta addition product (i.e., 2-phenylethyltrichlorosilane) is formed exclusively, so that the method described therein seems to offer a solution of the problems.
This exclusive formation of 2-phenylethyltrichlorosilane, however, is accomplished only if low temperatures and low transformation rates are used. If, however, this procedure is performed at temperatures upward of about 50.degree. C., again considerable amounts of 1-phenylethyltrichlorosilane are obtained, especially when high rates of input are used, such as are necessary for technical purposes to achieve large yields per unit of time.
The problem therefore existed of managing the platinum-catalyzed addition of hydrogen chlorosilanes onto styrene such that the formation of 1-phenylethylchlorosilanes and polymers of styrene will be suppressed, even when the reaction is performed at temperatures up to 100.degree. C. at high rates of throughput of the starting products.